1. Field of the Invention
This invention relates to a type of atomizer which atomizes liquid fuel by utilizing the kinetic energy of an atomizing medium such as high-pressure air or steam, and more particularly to an atomizer suitable for atomizing coal-water slurry (coal water mixture: hereinafter referred to as "CWM"). This invention also relates to a coal-water slurry fired boiler having an atomizer and a burner suitable for atomizing coal-water slurry by using an atomizing medium such as high-pressure air or steam.
2. Description of the Prior Art
One technique of fluidizing coal is concerned with CWM which is a new way of utilizing coal in place of dust coal and is now increasingly used in boilers for generating electric power or for industrial purposes. Since the use of CWM enables coal to be supplied to a burner by a pump as in the case of a conventional petroleum fuel, it is more favorable in terms of the manner in which fuel is conveyed as compared with air transportation of dust coal; specifically, it facilitates simplification of the flow control system and reduction of the diameter of fuel feed pipes. In examples of the recent usage of CWM, the concentration of coal in the CWM is increased to about 60 to 70 wt % in order to increase the calorific value to a level commensurate with its use as a boiler fuel.
CWM is not so readily burnt as petroleum, and the nitrogen content in CWM fuel is large. For this reason, the burn-off time of atomized particles of CWM is so long that it is difficult to form stable flames. Moreover, when the fuel is burnt, the rate of generation of nitrogen oxides (NOx) caused by the oxidation of the N fraction of the fuel is high. To effect good combustion of CWM fuel, it is necessary to promote heating of atomized particles by promoting the micronization of the fuel so as to reduce the size of atomized particles and hence the surface area thereof relative to the weight, and at the same time to reduce the time which it takes for combustion of the atomized particles.
To reduce NOx, it is necessary to form stable flames by improving the igniting performance, release the N fraction of the CWM before combustion air is fully mixed therewith, form a reducing atomosphere, and mix in the combustion air gradually. The promotion of micronization for good combustion and the promotion of the ignition and stabilization performance for reducing the generation of NOx act upon each other synergistically.
A two-fluid type atomizer which uses high-pressure air or steam for effecting micronization is employed to atomize CWM into fine particles. To micronize a fuel which is inferior in terms of combustibility, it is necessary to reduce the flow rate of the atomizing medium and hence the velocity of atomized particles while reducing the size of the atomized particles. For the purpose of obtaining a good mixture of fuel and atomizing medium, a type of atomizer which has an inner mixing chamber for mixing CWM with an atomizing medium and a plurality of holes provided as spray holes and formed in a wall of the mixing chamber has been disclosed in some known proposals, for example, Japanese Patent Laid-Open No. 36811/1985. In the atomizer disclosed in these known proposals, a chamber for preliminarily mixing fuel with an atomizing medium (hereinafter referred to as "fore mixing chamber") is formed at the upstream side of the mixing chamber. In this arrangement, fuel that has been once changed into fine particles is supplied to the mixing chamber, and mixing of the fuel with the atomizing medium in the mixing chamber is promoted, whereupon it is jetted as atomized particles through spray holes.
The atomizer disclosed in these known proposals is capable of providing fine atomized particles since the atomizing medium can be suitably mixed in the fore mixing chamber under the condition that the mass flow of the atomizing medium relative to that of the fuel is high enough to change the fuel into fine atomized particles.
However, under circumstances that the atomizing medium/liquid mass ratio is small, the momentum of the fuel is greatly increased compared with that of the atomizing medium. For this reason, after the atomizing medium has been jetted properly in the direction that it is supposed to be jetted, it is deflected by the momentum of the fuel toward the downstream side of the pre-mixing chamber, that is, toward the mixing chamber. With respect to the local atomizing medium/liquid mass ratio in the pre-mixing chamber, an atomizing medium/liquid mass ratio higher than that observed when fluids flow in the atomizer is displayed beside the wall of the pre-mixing chamber while an atomizing medium/liquid mass ratio lower than the observed ratio is displayed in the central region of the pre-mixing chamber. Under this condition, the fuel is changed into large particles in the central region of the pre-mixing chamber and into fine particles near the wall thereof. As a result, large particles are supplied to the mixing chamber despite the provision of the pre-mixing chamber and micronizing characteristics are therefore poor.
When the sectional area of the pre-mixing chamber is reduced for the purpose of limiting the generation of large particles, CWM becomes clogged in the mixing chamber, thereby obstructing the action of micronization.
In a suitable form of CWM combustion, fuel is jetted under air-deficient conditions, and mixing of the combustion air with the fuel is positively promoted, thereby improving the ignitability and preventing recession of flames so as to achieve low-NOx combustion. Japanese Patent Laid-Open Nos. 202402/1982 and 19929/1983 disclose examples of a burner in which the mixing of atomized fuel and combustion air is promoted so as to improve the ignitability. In these examples, a fuel pipe is disposed on the axis of the burner, and a plurality of air jet orifices are formed around a fuel jet opening through which the fuel is jetted from the fuel pipe such as to be coplanar with this opening, the fuel pipe and air ejection orifices being provided in a wall of the burner tile which faces the opening formed on the side of the furnace. This type of burner has improved characteristics in terms of ignitability because they are capable of promoting the process of mixing fuel and air and heightening the temperature in the burner tile under the condition that the fuel jet speed is comparatively low, i.e. about 10 to 30 m/s, as in the case of pulverized combustion. However, this burner displays atomized CWM speeds of 100 to 200 and several 10 m/s which are generally five times as high as in the case of pulverized, so that a negative pressure is caused in the vicinity of the outer periphery of the flow of atomized CWM, thereby generating counterflow circulations flowing from the outside of the burner tile to the inside thereof. Since air flows in the form of a plurality of jets along the circumference of the flow of atomized CWM, the pressure distribution in the vicinity of the periphery of the high-speed flow of atomized CWM is not uniform, resulting in unstable and non-uniform counterflow circulations. There is therefore a problem of collision of the flow of atomized CWM against the wall of the burner tile.
CWM contains a great deal of ash. For this reason, when atomized CWM collides with the wall of the burner tile, ash is solidified on the surface of the wall, the pressure distribution in the burner tile becomes less and less uniform, and the possibility of collision of CWM hence increases further, thereby causing ash to adhere to the internal part of the burner tile.
It is therefore important in the combustion of CWM to prevent atomized CWM from sticking to burner walls such as a burner tile as well as to promote the mixing of combustion air.
Japanese Patent Laid-Open No. 145405/1984 discloses a further example of a conventional burner which is provided with a burner tile and in which combustion air is supplied in the form of swirling flows from a wall which faces the opening of the burner tile. However, in this burner, the rate of attenuation of the flow velocity of atomized CWM is reduced such as to cause a problem of recession of the igniting position when combustion air is jetted. This is because combustion air flowing in this type of burner has a velocity component in the axial direction due to the form of construction of the burner and, in general, the higher the relative velocity between a jet and flows flowing along the jet, the larger the rate of attenuation of the jet velocity.
These known arts are based on a policy of independently improving the performance of both the atomizer and the burner. They lack consideration of possible improvements in the combustibility of liquid fuel and reduction of NOx generation from the viewpoint of an overall burner system design including both the burner and the atomizer. In these known arts, not only is combustion air not mixed in a manner proper to the state of atomized particles, but the promotion of micronization for the achievement of good combustion and the promotion of ignition and flame-stabilization are not effected synergistically.
As described above, conventional atomizers generate large-size particles (generally having a diameter of more than 100 .mu.m) because CWM fuel and the atomizing medium are not mixed in a suitable manner. The large-size particles display a small rate of velocity attenuation and are expelled from the furnace after a very short residence time without any involvement in the combustion, resulting in accumulation of the unburnt part.
If a dilatant fluid which increases the rate of generation of large-size particles is provided as a liquid fuel, it is necessary to use a type of atomizer capable of micronizing fuel independently of the flow characteristics.
In the conventional burners having a burner tile for promoting the mixing of atomized CWM with combustion air, air jetted into the burner tile has a velocity component in the direction of the jet of atomized particles, so that the rate of velocity attenuation of atomized particles is obstructed, thereby causing a problem of flames retreating from the burner.
Air which is supplied from the outer periphery of the burner tile is necessarily mixed with atomized fuel in order to prevent the recession of flames. This is one reason for the difficulty in reducing NOx generation.